1. Field of the Invention
This invention relates to a method and apparatus for determining time differences between two or more platforms, for synchronising platforms, for determining the time of arrival of a signal at platforms for locating an emitter, and more particularly for comparing the difference in time of arrival of a signal at two or more platforms.
2. Description of Related Art
Radio Frequency (RF) emitters, such as radars have previously been located by single platforms using angle of arrival information. The platform measures the angle of arrival of an emission while logging the current position of the platform from navigational data. Another angle of arrival measurement is made from a second position. The intersection of the two angles of arrival provide the position of the emitter. However, this method has limitations due to angular resolution limits and errors in position measurements. The angular resolution limit implies that the emitter lies on a given bearing within a tolerance in azimuth, i.e. the emitter lies within an arc. With an angular measurement covering azimuth and elevation, the intersecting error pattern gives an error volume in which the emitter lies. The error volume is known relative to the platform position at the times of measurement. The logged positions may not be the actual positions of the platform due to errors in the navigational system.
As a significant amount of time elapses between the measurements of the emission angle of arrival, the emitter could move between measurements, so increasing the error in the emitter location.
To avoid such problems with platform movement, location measurement needs to be performed quickly or nearly instantaneously. Multiple geometrically-separated platforms can therefore use 2D angle of arrival information to locate the position of an emitter within the angular error of such a system.
An alternative technique has been used with multiple platforms to resolve location from Time Difference of Arrival (TDOA) measurements. A typical TDOA system uses a number of spatially separate transmitters which are synchronised in time. Each transmitter transmits an identifiable pulse chain. The time difference between the arrival of the pulse chains from the different stations enables time difference of arrival hyperboles to be calculated for each pair of stations. A portion can be calculated from two intersecting hyperboles provided the positions of the transmitters are known. The velocity of the platform during measurement can, however, affect the accuracy of the system.
Similarly (in reverse) the position of an emitter can be located using three or more receivers which are synchronised in time and located at known positions by generating hyperboles representing the difference in arrival time of a pulsed signal at pairs of receivers.
The TDOA technique provides an improved location precision at long range than angular intersect techniques, with antenna arrays that are compatible with modern jet aircraft sizes.
Moving platforms are subject to relativistic effects which make it difficult to determine position and time accurately on multiple spatially-separate airborne platforms. For TDOA techniques to be viable on moving (3D) platforms, a space and time reference system is required. While a Differential Global Positioning Satellite (DGPS) system provides positional accuracy to the order of 0.6 metres on dynamic platforms, a method for determining time differences between two or more geometrically separate platforms is required to synchronise the platforms.
Traditional time transfer techniques are used to synchronise laboratories, for example. In this case, a laboratory transmits the real local time provided by its local clock to another laboratory via a geo-stationary satellite. A counter at each laboratory measures the time difference between the time indicated by its associated local clock and the time received from the remote clock associated with the other laboratory in order to determine a time offset between local and remote times. Fixed ground stations, off-line processing and long-time constants are, however, are usually necessary to conduct this process as with long range transmissions and atmospheric variations, a long term average is required to meet desired measurement accuracies. Furthermore it is assumed that the propagation paths are varying slowly with the time frame used.